The present invention relates to a method and apparatus for conditioning tobacco particles, and more particularly to a method and apparatus for conditioning tobacco particles in the form of whole leaves, laminae, ribs and stem and/or shreds which preferably form a continuous stream and are transported through a series of conditioning zones. Still more particularly, the invention relates to improvements in a method and apparatus for conditioning tobacco particles by means of a gaseous fluid, preferably air.
It is well known that many treatments of tobacco must be preceded by increasing its moisture content so as to enhance the flexibility of tobacco particles (e.g., prior to shredding) and to thus reduce the likelihood of undesirable breakage or comminution and the formation of dust. It is also known to treat tobacco with a variety of substance (known as casing) in order to improve its aroma; such treatment also involves increasing the moisture content of tobacco. At least some moisture must be expelled from tobacco prior to final processing into cigarettes, cigarillos, cigars, pipe tobacco, chewing tobacco and/or other types of smokers' products. For example, prior to feeding tobacco shreds into a modern mass-producing cigarette making machine, it is necessary to insure that the moisture content of tobacco shreds is invariably within an extremely narrow range which cannot deviate from an optimum moisture content by more than a small fraction of one percent. Heretofore known procedures which are employed in connection with conditioning of tobacco particles normally involve contacting tobacco with a gaseous fluid which is conveyed countercurrent to or concurrently with the direction of tobacco transport through the conditioning apparatus, or a combined concurrent-countercurrent treatment.
The countercurrent treatment involves conveying a current of hot air counter to the direction of transport of tobacco particles and across the tobacco stream. Such treatment is desirable in many instances because the tobacco is treated gently. This is due to the fact that the particles of tobacco are contacted first by a current of hot air at less than maximum temperature i.e., the hottest portion of the air stream comes into contact with that portion of the tobacco stream which was already treated by cooler air. In other words, the moisture content of tobacco particles is inversely proportional to the temperature of air which contacts successive increments of the stream. As the temperature of air decreases in response to continued contact and energy exchange with tobacco, the moisture content of air rises so that each increment of the tobacco stream is contacted first by an air current having a relatively high moisture content and a relatively low temperature; the moisture content of air decreases and the temperature of air increases in the direction of tobacco transport. Thus, the very hot air contacts those particles of tobacco whose moisture content is already reduced and the relatively cool air contacts tobacco particles whose moisture content is high. Otherwise stated, the maximum exchange of energy takes place at the downstream end of the conditioning zone where the partially conditioned tobacco is contacted by freshly admitted air whose temperature is high. An advanatage of such countercurrent conditioning is that the final moisture content of tobacco can be selected and maintained with a high degree of accuracy because, if the moisture content in a region slightly ahead of the downstream end of the conditioning zone deviates from the desired optimum moisture content, the temperature of freshly admitted hot air can be readily changed to compensate for such differences without any delay or with negligible delay. However, the just described countercurrent conditioning cannot insure that the moisture content of each portion of each tobacco particle is within the desired range, i.e., the overall moisture content of a batch of tobacco particles is satisfactory but the moisture content is likely to vary from portion to portion of a discrete tobacco particle.
The conditioning of tobacco with a current of hot air which is conveyed concurrent with the direction of tobacco transport involves conveying the tobacco in and often by the current of air. Such treatment renders it possible to achieve a hygroscopic equilibrium between tobacco and air at the downstream end of the conditioning zone, i.e., all portions of each tobacco particle will have a desired moisture content. However, the concurrent treatment exhibits a serious drawback, namely, that the tobacco particles are contacted first by very hot air which effects an abrupt drying of strata adjacent to the external surfaces of tobacco particles. As a result of such treatment, certain types of tobacco are likely to develop hard crusts. Moreover, the intervals between the measurement of moisture content and an effective adjustment in the event that the final moisture content of tobacco is unsatisfactory are very long. Thus, if the moisture content of tobacco is measured downstream of the conditioning zone and if such moisture content is unsatisfactory, it takes a relatively long interval of time to change the moisture content by changing the characteristics of the air current. This means that a substantial amount of tobacco is allowed to leave the conditioning zone with a final moisture content which is unsatisfactory for further processing. Also, the regulating system which is used to change the characteristics of an air current flowing concurrent with the direction of tobacco transport is likely to begin to oscillate. Since the main drying action takes place in the relatively short foremost portion of the conditioning zone, it is difficult to influence the moisture content of tobacco in the remaining portion of the conditioning zone if such moisture content is unsatisfactory.
The combined concurrent-countercurrent conditioning of tobacco exhibits the advantage that the adverse effects of the preceding (concurrent) treatment can be compensated for during the next-following (countercurrent) treatment. However, this takes place at the expense of uniformity of moisture content in all portions of tobacco particles which leave the conditioning zone. Thus, the countercurrent treatment which follows the concurrent treatment is likely to reduce the uniformity of moisture content in each portion of a tobacco particle which leaves the treating apparatus. Furthermore, even though the concurrent treatment allows for convenient elimination of substantial deviations of measured moisture content from a desired moisture content by proper regulation of the temperature of air currents which are transported concurrent with tobacco particles, such concurrent treatment is likely to bring about undesirable incrustation of tobacco particles in that portion of the conditioning zone where a current of very hot air comes into contact with tobacco. Therefore, conditioning apparatus whose operation is based on the just discussed principle have failed to find widespread acceptance in the tobacco industry.
Freshly gathered tobacco leaves are normally dried at the farm. Prior to compacting of leaves in barrels, hogsheads or in the form of bales (this is the customary form of compacting tobacco for shipment to manufacturing plants), the moisture content of tobacco leaves must be reduced to an accurately determined relatively low value. Such accurate drying cannot be achieved by mere exposure of tobacco leaves to atmospheric air because various portions of tobacco leaves lose their moisture content at a different rate. Thus, the laminae will dry much faster than ribs and stem. Therefore, freshly gathered tobacco leaves are often stemmed or destalked to separate laminae from stem so that the thus separated stem and laminae can be dried independently of each other. Tobacco which is subjected to a drying action after gathering is often called greenleaf tobacco. Accurate drying of greenleaf tobacco (so that the moisture content of each tobacco particle and of each portion of each tobacco particle remains within a narrow range) is desirable and necessary because, after the tobacco is compacted in barrels or in the form of bales, batches with a higher moisture content tend to mildew and the molding spreads very rapidly throughout an entire barrel or bale to cause substantial damage. A contemporary drying apparatus for greenleaf tobacco normally comprieses a rotary open-ended vessel or barrel through which a stream of tobacco is conveyed with a current of air with attendant agitation of tobacco to insure more satisfactory exchange of energy between air and tobacco particles. It is also known to employ drying apparatus in the form of pneumatic conveyors wherein greenleaf tobacco is conveyed in a current of hot air passing through a pipe or the like.
Since a bale or barrel of dried greenleaf tobacco is likely to remain in storage for extended periods of time, the manufacturers require that the moisture content of such tobacco be maintained within an extremely narrow range because a very small nest of mildewy tobacco is likely to contaminate the contents of an entire barrel or an entire bale before the barrel or bale is removed from storage. As mentioned before, the moisture content must be reduced to a very low value, and the transport through a drying or conditioning apparatus wherein the moisture content is reduced to such low value is likely to result in undesirable breakage or comminution of tobacco and/or in the formation of excessive quantities of tobacco dust. The likelihood of breakage or dust formation is particularly pronounced in the aforementioned drying apparatus wherein a stream of tobacco is conveyed through a revolving open-ended vessel in the presence of hot air. This is due to the fact that the revolving vessel subjects the particles of tobacco to a very pronounced agitating action during which the particles are repeatedly lifted well above the bottom zone of the vessel by orbiting rakes or blades and are allowed to drop by gravity back into the bottom zone. Nevertheless, many tobacco growers still employ such types of drying apparatus because their conditioning action is more uniform than that of pneumatic drying apparatus.
Additional problems in connection with conditioning of tobacco arise due to the fact that the space in a modern tobacco processing plant is at a premium. Thus, the manufacturers strive to accommodate the conditioning apparatus in a small area while simultaneously desiring a high output and an accurate conditioning of each and every tobacco particle, i.e., the temperature and/or moisture content of conditioned tobacco should equal or should be very close to an optimum temperature and moisture content. This can be achieved if the dimensions of conditioning apparatus can be increased at will, i.e., if the drying of moist tobacco can be carried out while the particles of tobacco are advanced at a relatively low speed and through a relatively long conditioning zone so that the reduction of moisture content can be achieved gradually. In other words, a satisfactory reduction of moisture content and a satisfactory heating or cooling of tobacco presents no problems if the changes in such characteristics are effected at a slow rate, namely, if the extent of drying per unit length of the conditioning apparatus and per unit of time is relatively small. Such treatment insures reliable homogenization of the final product so that the temperature as well as the moisture content of each particle issuing from the conditioning zone is best suited for further processing. This is due to the fact that a conditioning for a long period of time and in a relatively long conditioning zone can readily compensate for fluctuations in initial temperature and/or moisture content as well as for eventual fluctuations in the intensity of treatment during one or more drying stages.
In certain presently known tobacco conditioning apparatus, a relatively short heating zone is followed by a homogenizing zone whose function is to eliminate eventual variations in moisture content of tobacco which issues from the drying zone. Such procedure is satisfactory only if the initial moisture content of tobacco (i.e., of tobacco which enters the drying zone) is constant or deviates only slightly from a fixed value. It has been found that the just described conditioning apparatus fail to insure a satisfactory homogenization and cannot achieve a reduction of moisture content to a fixed value unless the initial moisture content is constant or deviates only negligibly from a fixed value. Thus, fluctuations of initial moisture content are likely to adversely influence the drying action or to prevent a satisfactory drying to such an extent that the homogenizing or uniformizing treatment cannot cure the defective drying action.